Pros/Cons of Working at Big R&D Consulting Firm?

pagalvin asks: "I'm being recruited for an 'R&D Architect' position at a Big 4 consulting firm in the U.S. Does the community care to share its experience working as 'overhead' in a large organization that is most famous for its consultants working 60 hour weeks and billing 'til the fat lady sings? In such places, do non-billable R&D types get any respect? Is there a a long-term career path that sticks with the technology track?"